A filter assembly for a plastic-molding apparatus typically has an inlet plate formed with an inlet passage having an upstream end adapted to receive the stream of pressurized molten plastic from the extruder and a downstream end opening at downstream port, an outlet plate formed with an outlet passage having a downstream end and an upstream end opening at an upstream ports aligned with the downstream port. A rotatable filter disk engaged between the plates is formed with a circularly annular array of throughgoing holes each defining a respective seat and the ports and holes being laterally spaced from each other such that in predetermined positions of the disk the ports open into two adjacent holes. Each seat holds a respective filter unit normally comprises of a perforated plate having an upstream side covered by a filter medium, such as a mesh or cloth.
With such a system the filter disk can be rotated to move one filter unit out of the flow path and another into the flow path without interrupting the flow. The advantage of this is not only that flow can continue unabated, but that the pressure difference before and after the changeover will be nominal. Since, presuming that filters are switched at regular intervals, the filter unit that is moved out has been in service twice as long as the one remaining in service, the change in back pressure across the filter units will not change as drastically as if clogged filter units were changed with clean ones.
As a rule the filter units are generally kidney-shaped so that they can nest closely with one another on the filter disk, only separated by thin webs. The disadvantage of this construction is that the invariably circular-section passages and ports do not actually cover whole filter units, so flow through them is invariably partial. Furthermore the construction of both the filter disk and of the filter units themselves is fairly complex due to the irregular shapes of the units and the seats they must fit in. As a result the filter assembly is an expensive piece of equipment.
While it is known from the in-house publication "Siebwechseleinrichtungen zur Verarbeitung von Thermoplasten" (sieve-changing devices for working on thermoplastics) of Machinenfabrik Joachim Kreyenborg & Co. of Munster, Germany, to have a branched passage system, in this arrangement flow is switched from one branch to the other during filter changeover. Thus flow is at least part of the time only through one filter unit, resulting in wide pressure fluctuations.